A communication system has a power amplifier, included in a transmitter, for amplifying transmission signals during signal transmission, and the transmitter power-amplifies transmission signals by means of the power amplifier and transmits the power-amplified signals.
Currently, communication systems are increasingly evolving, and due to the evolution of communication systems, the transmitter increasingly requires use of broadband signals. In addition, communication systems require a high Peak-to-Average Ratio (PAR) of transmission signals according to signal transmission. Therefore, a power amplifier included in the transmitter requires higher linearity and higher efficiency.
Power amplification schemes used for the power amplifier are classified into a Doherty scheme, an Envelope Elimination and Restoration (EER) scheme, a Delta-Sigma Modulation (DSM) scheme, and a Linear amplification using Nonlinear Component (LINC) scheme.
Of the power amplification schemes, the Doherty scheme and the EER scheme are superior in terms of efficiency, and the DSM scheme and the LINC scheme are superior in terms of linearity. Further, the EER scheme uses signals of polar components without using rectangular (IQ) signals (i.e, signals having a 90°-phase difference between an In-phase (I) component and a Quadrature (Q) component), which are represented by I and Q components. The EER scheme uses, as an input to a power amplifier, phase components of signals having a constant envelope after undergoing envelope elimination. In addition, the EER scheme modulates direct current (DC) bias of a switch mode or a saturation power amplifier for envelope restoration. With reference to FIG. 1, a description will now be made of the EER scheme.
FIG. 1 is a diagram illustrating a structure of a transmitter using an EER scheme in a conventional communication system.
Referring to FIG. 1, the transmitter includes a baseband signal generator 111, a rectangular-to-polar converter 113, a bias modulator 115, a phase modulator 117, and a power amplifier 119.
The baseband signal generator 111 generates rectangular (IQ) signals of a baseband using voice signals or data.
The rectangular-to-polar converter 113 converts the rectangular signals into polar signals which are divided into amplitude components and phase components.
The bias modulator 115, which supplies power (i.e., voltage or current) to a power amplifier, supplies voltage or current components corresponding to signals of the amplitude components to the power amplifier 119.
The phase modulator 117 performs phase modulation to generate radio frequency (RF) signals of a desired frequency band using the signals of the phase components.
The power amplifier 119 receives the signals phase-modulated by the phase modulator 117, amplifies the power of transmission signals, and combines amplitude components depending on a change in a bias voltage by the amplitude components generated by the bias modulator 115 to thereby restore the original signals. Since an input to the power amplifier 119 has a constant envelope and there is no distortion for an input signal even though a switch mode or a saturation power amplifier is used, the power amplifier 119 has a high-efficiency characteristic. With reference to FIGS. 2A to 2C, a description will now be made of internal signals of a transmitter using the EER scheme.
FIGS. 2A to 2C are graphs illustrating internal signals of a transmitter using an EER scheme in a general communication system.
Before a description of FIGS. 2A to 2C is given, it is assumed that baseband signals used in the present invention use sine waves of a low-frequency band. In each graph, the horizontal axis represents time, and the vertical axis represents the amplitude of signals. For example, referring to FIG. 2A, shown is a sine wave signal having an amplitude ‘1’ and a period ‘1’.
Referring to FIG. 2A, shown are an output signal (a) of a baseband signal generator in a transmitter, and an output signal (b) corresponding to an amplitude component obtained by converting a polar of the signal ‘a’ by a rectangular-to-polar converter in the transmitter.
The signal ‘a’ is a low-frequency sine wave signal, and the signal ‘b’ is an envelope signal, which is an amplitude signal of the signal output from the rectangular-to-polar converter.
Referring to FIG. 2B, shown are an output signal (c) corresponding to a phase component obtained by polar-converting the signal ‘a’ by the rectangular-to-polar converter in the transmitter, and a signal (d) obtained by phase-modulating the signal ‘c’ by a phase modulator in the transmitter. The vertical axis of the graph showing the signal ‘c’ represents a phase. The signal ‘d’ is a phase signal which is modulated with a carrier signal having a frequency which is approximately 10 times higher than a baseband signal of low-frequency sine waves.
Referring to FIG. 2C, shown are an abnormal output signal (e) of a power amplifier in the transmitter, and a normal output signal (f) of the power amplifier.
The signal ‘e’ is a signal distorted due to the band limit, and the signal ‘f’ is a signal that should be normally output from the power amplifier.
Since the transmitter using the EER scheme converts the rectangular signal into the polar signal, the amplitude components and the phase components have more high-frequency components than the original signals. Therefore, the bias modulator should have an operating bandwidth which is, for example, 2 to 5 times that of the baseband signal, and the power amplifier should have an operating bandwidth which is, for example, 5 to 20 times that of the baseband signal. Thus, the transmitter using the EER scheme increases the operating bandwidth of each hardware module in the transmitter.
In addition, as the operating bandwidth increases, the transmitter using the EER scheme increases in non-linearity caused by the transmission signal's power amplification due to the limitation on implementing a digital-to-analog converter (DAC) and the limitation on implementing the bias modulator because of the limit and loss of switching frequency of the switching elements.